Anaerobic digestion is a series of processes in which organic material is dissolved and chemically converted in the absence of oxygen so that it can be absorbed by the cells of an organism and used to maintain biological functions. During anaerobic digestion, complex carbohydrates (e.g., cellulose and starch), lipids, fibers, and proteins, are converted into simpler compounds (e.g. sugars, glycerin and fatty acids, and amino-acids) which can be taken up by cells. Conversion occurs due to reduction of the complex organic compounds by hydrolytic enzymes, such as cellulases, proteases, and lipases, secreted by bacteria, which split the long molecular chains into monomer units.
The process of commercial anaerobic digestion generally employs specialized bacteria to break down organic waste as described above, and then to convert it into biogas (a mixture of carbon dioxide and methane) and a stable biomass. Under anaerobic conditions, a considerable portion of the chemical oxygen demand (COD) is converted to methane gas as an end product. Methane is a potential energy source, and its production from waste considerably lessens waste biomass disposal requirement and the financial burden associated with disposal. Biogas produced from anaerobic digestion has thus been promoted as a part of the solution to energy problems. Methane has a calorific value of 9000 kcal/m3, and can be burned on site or elsewhere, for example, to provide heat for digesters or to generate electricity.
Solid waste and other biodegradable solid substrates should be handled, as much as possible, in a manner that reduces their environmental impact, recovers energy locked therein, and avoids massive disposal treatments (e.g., landfill, incineration, etc.). Most of the anaerobic digestion technologies that are currently applied to domestic wastewater treatment, dairy and swine manure, and food processing waste can handle up to 10% total solids (TS). Application of these existing technologies to process high solids (>10%) streams often require significant dilution, larger digester sizes and high fresh water consumption, resulting in very high capital investment costs. High solids digestion technologies for municipal solid waste treatment have been developed and applied more extensively in Europe than in the US. However, these technologies depend on significant recycling of the treated solids to maintain the bacterial population in the digester, which requires additional reactor volume and expensive equipment.
The treatment of solid waste using anaerobic digestion poses several challenges because of the variety in the feedstock and the space limitations where such facilities can be located. For example, the organic fraction of municipal solid waste (OFMSW) may contain agricultural, food, yard waste, and/or paper in varying concentrations, sizes, and compositions. Furthermore, municipal solid waste is contaminated with non-organics, such as glass and metal, and therefore requires pre-treatment to separate these from the feedstock. Though the ideal waste stream for an anaerobic digestion plant would be source-separated organics, the reality is that there is always a small degree of contamination that must be handled on site, and additional equipment is usually needed to remove this contamination prior to digestion in existing anaerobic digestion systems.
Preferred designs of anaerobic digestion systems reflect the need for shorter hydraulic retention times (HRTs), higher retention of biomass, smaller reactor volumes and higher loading rates, indicative of their urban locations. U.S. Pat. No. 4,735,724 (Chynoweth, et al.) describes a non-mixed vertical tower anaerobic digester for accommodating high solids loadings and providing separation of microbial phases within the continuous digester volume to achieve substantially complete bioconversion of biodegradable feedstock components. Due to the passive concentration of solids in the upper portion of the reactor, biodegradable solids have an increased retention time in the digester, whereas liquids and non-biodegradable components have a reduced retention time, since they migrate to lower portions of the digester and are withdrawn preferentially. Non-mixing allows this single digester to be operated at high solids loading because passive concentration of solids and the separation of microbial phases within a continuous digester volume results in greater system stability. However, overall kinetics of the degradation process and therefore biogas productivity as well as yield is reduced, primarily as a result of the passive mixing and overall high loading.
Separated two-stage anaerobic digestion processes, where the acid stage digestion and the methane stage digestion are carried out in two separate reactor vessels, have been found to enhance the efficiency of conversion of organic carbonaceous materials to methane. The main disadvantage is the cost of such more complex systems. Two-stage anaerobic digestion of organic carbonaceous materials to produce methane is generally taught by U.S. Pat. No. 4,022,665, U.S. Pat. No. 4,318,993, and U.S. Pat. No. 4,696,746 (all to Ghosh, et al). Each of these patents teaches performing acid stage digestion and methane stage digestion in two separate reactor vessels. Each of these patents also teaches operating conditions for acid stage and methane stage digestion. U.S. Pat. No. 5,500,123 (Srivastava) describes operating conditions for a two-stage anaerobic digestion process, such as feed rates and retention times, and teaches introduction of oxygen into the methane phase digester to produce biogas having a methane content in excess of 80%.
U.S. Pat. No. 6,342,378 (Zhang, et al. and U.S. Pat. No. 7,556,737 (Zhang) teaches methods and a device for the generation of methane by a two-stage anaerobic phase system (APS) digestion of organic substrates. The APS-digester system is a space-efficient, high-rate solids digestion system. The APS-digester system consists of one or more hydrolysis reactors and one biogasification reactor. The hydrolysis phase, the buffer tank and the methanogenesis phase are operative over variable pH ranges that are related to the nature of the organic substrate and the amount of total solids in the organic substrate. In a preferred embodiment, the pH of the hydrolysis reactor is maintained in the range of from about 4.5 to about 7.0. In another preferred embodiment, the biogasification stage pH is maintained in the range of from about 6.5 to about 8.0. Compared with the other two-stage systems in U.S. patents, the APS-digester can process higher total solids organic waste streams. The microorganisms in the hydrolysis reactors are selected and environmental conditions are controlled to allow production and release of hydrogen in the first stage prior to methane production in the second stage.
Such prior art two-stage systems are best suited for degrading highly cellulosic feedstock such as rice straw, forestry waste, agricultural waste, and water and land plants or organic carbonaceous material with low total solids such as sewage sludge, municipal waste, and animal waste. If these two-stage systems are applied to easily degradable high solids waste such as food waste or animal waste, the main advantage would be producing relatively high hydrogen gas content in the first step but at the cost of no methane production in the first step and low methane production in the second step. However, current hydrogen production from anaerobic digestion is not economically viable because of the high cost required to purify the hydrogen gas content from 30% to over 98% to meet the hydrogen quality standards such as Title 13 CCR Section 2292.7-1995, JIS K-0512 Type 3 and MIL-PRF-27201C. Furthermore, the previous two-stage systems are either simple but inefficient, or efficient but complicated.
There is a need in the art for simplified, efficient and cost effective anaerobic digester systems for high solids waste streams.